1. Field of the Invention
The present invention relates to a modulator for data transmission between a transponder and a base station, which amplitude modulates and/or phase modulates a received electromagnetic carrier signal as a function of data to be transmitted. The invention further relates to a transponder having such a modulator, and a modulation method for operating such a modulator.
2. Description of the Background Art
The invention resides in the field of transponder technology and more particularly in the field of contactless communication for the purposes of identification. Although applicable in principle to any desired communication systems, and thus to any desired modulation devices, the present invention and the problems it was designed to solve are explained below with respect to so-called RFID communication systems, and in particular to RFID modulation devices and their applications. In this context, RFID stands for “Radio Frequency Identification.” In RFID systems, data are transmitted wirelessly between one or more base stations (or writing/reading devices) and one or more transponders. For general background on such RFID technology, please refer to “RFID Handbuch” by Klaus Finkenzeller, Hanser Verlag, third revised edition, 2002.
Passive transponders have no independent energy supply, while semi-passive transponders have their own energy supply, but neither active nor semi-passive transponders have an active transmitter for data transmission to the base station. In passive and semi-passive RFID systems of this nature, the backscatter technique is generally used for data transmission—especially at distances of significantly more than one meter—in conjunction with UHF or microwaves. In the backscatter technique, the backscatter cross-section of the antenna is used for return data transmission from the transponder to the base station. To this end, modulated electromagnetic carrier signals are transmitted by the base station, and are received and demodulated by a transmitting and receiving device in the transponder. In addition, the electromagnetic carrier signals are modulated by the transponder with a customary modulation method as a function of the data to be transmitted to the base station and are reflected. In general, this is accomplished by a change in the input impedance of the transponder's transmitting and receiving device, which causes a change in the reflection properties of an antenna connected thereto. In addition to amplitude modulation (ASK), methods used for modulation in modern communications systems increasingly also use phase modulation (PSK) and frequency modulation (FSK) in conjunction with pulse-width modulation. Various methods are known for this purpose.
In a first type of modulation method, which is described in European patent application EP 1 211 635 A2, which corresponds to U.S. Pat. No. 6,870,460, the real component of the input impedance is changed by connecting and disconnecting an essentially ohmic, and hence resistive, load, which primarily achieves an amplitude change or amplitude modulation of the reflected waves. This modulation method is called amplitude shift keying (ASK). The ohmic load here loads the voltage supply of the transponder as an additional consumer; as a result, the maximum distance that can be spanned between the transponder and base station is significantly reduced, especially for passive transponders without their own energy supply. ASK modulation is thus especially suitable for short distances between the base station and transponder, but is only usable to a limited extent for greater distances.
In a second type of modulation method, the imaginary component of the input impedance is influenced by changing the capacitance of a capacitor in the input section of the transmitting and receiving device, primarily achieving a phase change or phase modulation of the reflected waves. This modulation method is called phase shift keying (PSK). A method of this nature is disclosed in the applicant's German patent application DE 101 58 442 A1, for example. In comparison to ASK modulation, PSK modulation has almost no effect on the operating voltage, with the result that a higher efficiency is achievable for the transponder and the maximum spannable distance between the transponder and base station increases. However, the power reflected by the transponder may decrease when the distance between the transponder and base station is reduced. At very small distances, the situation can arise that the base station can no longer detect the phase modulated signal reflected by the transponder. Thus, PSK modulation may not be optimal for very short distances between the base station and the transponder.
The need thus exists to be able to operate an RFID system in both the near field region, which is to say at relatively short distances between transponder and base station, and the far field region, which is to say at relatively long distances between transponder and base station. Please refer to the aforementioned RFID Handbuch by Klaus Finkenzeller for definitions of near field and far field regions.
In prior art modulators in RFID communications systems, a switching unit intervenes either before the rectifier (in the case of ASK and PSK modulation) or after the rectifier (in the case of ASK modulation as load modulation). In German patent application DE 103 01 451, which corresponds to U.S. Pat. No. 7,151,436, which is herein incorporated by reference, a method is described in which a multi-stage rectifier arrangement is provided and the modulator intervenes at a shared node located between adjacent stages of the rectifier. In this context, the arrangement achieves an amplitude modulation in that modulation of the carrier wave transmitted back (returned) to the base station is accomplished by a switching means connected to the rectifier. To this end, a modulation control signal is supplied to the switching means. This solution places a considerable load on the switching unit of this DC node, and in particular can only be used with passive, especially backscatter-based, transponder systems when high energy absorption by the transponder from the electromagnetic field is possible in the near field, i.e. at very short distances.